High viscosity heat sensitive ink printing process

ABSTRACT

A high viscosity aqueous inkjet ink that is useful in ink jet printers. The ink comprises heat activated colorant solids that are not heat activated during the printing process, and are printed onto a substrate in the form of an image that can be transferred onto a subsequent or final substrate by applying heat and intimate contact between the two substrates. The ink can also be heat activated onto the substrate without further transfer by applying heat at the temperature that is suitable for the activation of the colorant.

FIELD OF THE INVENTION

This invention relates to a method of inkjet printing of heat activatedcolorants and an ink comprising heat activated colorants.

BACKGROUND OF THE INVENTION

Inkjet digital printing has been widely used in many applications. Itsrefined image quality, comparing with conventional analog technologiessuch as offset printing, screen printing, lithographic printing, digitalinkjet printing technologies generate much convenient, efficient andenvironmentally clean results.

However, aqueous digital printing on materials other than paper may beinferior to other processes due to lack of color intensity and speed dueto some shortcomings of digital inkjet printing methods. Among those,low viscosity, low colorant content aqueous inks with especially smalldroplet size requires much more ink materials to generate comparablefinal image output. This is greater problem when the colorants used inthe inkjet ink are not soluble dyes, but are pigments or other insolublecolorants. High color saturation and so called super saturation of thecolor image may be difficult to achieve when using these inkjet inks.

One means of creating aqueous ink is adding viscosity control agentsthat are high molecular weight natural synthetic polymers, higherviscosity water soluble or miscible glycols, high alcohols, accompaniedby higher concentrations of colorants. Several problems are associatedwith such simple approach. The use of high molecular weight polymers mayresult in an aqueous system that deviates from physical propertyrequirements of the inkjet printer used to print the ink, such asNewtonian fluid behavior. Therefore, the ink improperly responds to thejetting mechanism. The high molecular weight polymer in combination withincreased levels of colorants, especially non-soluble type of colorants,can create clogging of the print head nozzles, even when using newerprint head technology designed for higher viscosity inks, since suchprinters are not specifically designed for use with colorants that aresolids when printed.

Heat activated colorants have been used in digital inkjet printing. Theimage quality is dependent on how effectively and efficiently the heatactivated colorants are transferred or fixed to the substrate. Hale etal., U.S. Pat. No. 5,642,141 and Xu et al., U.S. Pat. No. 5,488,907teach inkjet printing methods using finely divided heat activated dyesolids. These methods incorporate an ink having a viscosity generallyaround 2 to 4 cP at ambient temperature. These patents do notspecifically teach one how to create a high transfer efficiency inkusing heat activated dyes and having relatively higher viscosity.

High viscosity inkjet inks may impose further problems for heatactivated inks when a high concentration of colorant is present. Agentsfor controlling or modifying physical properties of the ink may hinderthe heat activation efficiency of the colorant, due to high boilingpoints, affinity for the heat activated dye at the activationtemperature, or entrapment/encapsulation of the dye particle due to thelong polymeric chain structure of the chemical/agent. These issues maybe more pronounced when the particle sizes of the colorants used in theinks are very small. For example, a high concentration of glycerin mayalter the heat activation efficiency of a small dye particle undernormal heat activation temperature and duration. Further, a thickeningagent, such as carboxy methyl cellulose (CMC), may create anon-Newtonian system, while also hindering activation or sublimation ofthe heat activated dye.

Ink jet printer print heads, including Drop on Demand (DOD)piezoelectric print heads, have of nozzles and orifices of varyingsizes. These nozzles and orifices dictate droplet size, print speed, andjettable ink viscosity, and also the tolerance to non-soluble colorantsor polymeric particulates. An appropriate range of dye particle sizesbased on the nozzle or orifice size is important when formulating thehigher viscosity heat activated inks.

There is a need for a higher viscosity ink that comprises heat activateddye solids for digital printing, including transfer printing or directprinting, that will not clog the print head, will yield high heatactivation efficiency, and be environmentally safe, and which issuitable for high viscosity ink printers, which is printers that requireliquid inks having a viscosity of 5.0 centipoise or greater at ambienttemperature.

SUMMARY OF THE PRESENT INVENTION

The present invention is a high viscosity inkjet ink that is useful forprinting heat activatable images using ink jet printers such as Drop onDemand piezoelectric printers. The ink comprises heat activated colorantsolids that are not activated during the inkjet printing process, andare printed onto a substrate in the form of an image that can beactivated and transferred to a subsequent or final substrate by applyingheat and intimate contact between the two substrates. The ink can alsobe heat activated onto the substrate without further transfer byapplying heat at a temperature that is suitable for the activation ofthe colorant.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention is a liquid inkjet ink having a preferredviscosity of not less than 5 cP, and may have a viscosity from 6 to 100cPs, with total water content of not less than 30% by weight of thetotal ink formulation. A preferred viscosity range is from 7 cP to 30cP. A heat activated colorant is present in the ink as printed in theform of solid particles or particulates. The specific amount of thecolorant(s) is supplied to achieve proper color intensity and imagequality upon heat activation. The colorant is preferred to range from 1%to 15% by weight of the total ink formulation.

In one embodiment, the ink contains a substantial amount of viscositycontrol solvents/co-solvents such as diol, triol, glycols, polyol, highalcohol, amines, polyamine, amino oxide, etc. either alone or in mixtureincluding but not limited to: glycerin, ethylene glycol, diethyleneglycol, triethylene glycol, propylene glycol, dipropylene glycol,caprolactum, polyethylene glycol, polypropylene glycol, urea, sorbitol,2-pyrrolidinone, N-methylpyrrolidinone, polyvinylpyrrolidinone (PVP),polyvinylalcohol (PVA), gamma-butyrolactone (GBL),2-methyl-1,3-propanediol, polyethylene polyamines, etc. These are highviscosity water soluble or water miscible solvents/co-solvents that willnot substantially change the hydrophilicity of the ink system, but whichincrease the viscosity of the ink. These materials have little to notendency to solubilize the heat activated colorants. Furthermore, thesesolvents, co-solvents, or viscosity control agents will notsubstantially alter the Newtonian fluid behavior (incompressible) of theaqueous system. This behavior may be expressed by the followingequation:

$\tau_{ij} = {\mu ( {\frac{\partial u_{i}}{\partial x_{j}} + \frac{\partial u_{j}}{\partial x_{i}}} )}$

with comoving stress tensor P (also written as σ)

${\mathbb{P}}_{ij} = {{{- p}\; \delta_{ij}} + {\mu ( {\frac{\partial u_{i}}{\partial x_{j}} + \frac{\partial u_{j}}{\partial x_{i}}} )}}$

where,

-   -   τ_(ij) is the shear stress on the i^(th) face of a fluid element        in the j^(th) direction    -   u_(i) is the velocity in the i^(th) direction    -   x_(j) is the j^(th) direction coordinate

Other materials which may be used to adjust viscosity are polypeptidessuch as abietoyl soy polypeptide, undecilenoyl soy polypeptide,alcohol/glycol soluble prolamine, ethoxylated fatty alcohol, ethoxylatedfatty amine, acrylic amide, 2-ethyl-oxazoline homopolymers, copolymerand/or terpolymers.

Depending on the specific viscosity requirement of the printer and/orprint head, viscosity control solvents/co-solvents may weigh at leastthree times as much, and up to twenty times as much, as the heatactivated colorant solids of the total formulation weight of the ink inorder to maintain both high viscosity and jettability. The totalviscosity control solvents/co-solvents may comprise between 15% and 60%of the total formulation by weight.

Other ingredients may be used for stabilizing the colorant, and for fineadjustment of the physical properties of the ink such as surfacetension, pH value, conductivity, and density. Further, non-heatactivated dyes and other colorants may be used in combination with theheat activated colorant(s) to enhance image quality and properties incertain applications. Self-dispersing colorants, and/or pre-stabilizedcolorants may also be used.

Aqueous inkjet inks having the desired viscosity may have differentstabilization requirements, due to physical property changes, such asink density, Brownian movement of the particulates, and electricconductivity. These changes impact the particulate size distributionprofile, and especially the upper limit of the particle sizedistribution requirement. The following empirical equation may be usedfor help selection of the upper particle size distribution limit, if theparticle size distribution does not deviate substantially from a normaldistribution model:

$\varphi \geq ( \frac{5D}{1 - f} )^{3}$

where,

-   -   Φ is the narrowest ink pathway inside the print head such as        nozzle/orifice diameter (in micron)    -   D is the particle size presented as 95% from total particle size        distribution (in micron)    -   f is the colorant weight fraction of the total formulation        weight (<1)        For example, a print head having 35 micron nozzle/orifice        diameter indicates a particle size, at 95% distribution, of        equal to or less than 0.62 micron in diameter, if the colorants        are 5% of the total weight. This formula helps in creating an        ink that will not clog the narrow path inside the print head.

Heat activated colorants suitable for use may include various dispersedyes or sublimation dyes that are activated or sublimed by applying heatto the printed substrate or transfer substrate. Generally, the heatactivation temperature does not exceed 450° F., and most preferably,does not exceed 410° F. Examples of colorants, in varying ratios,include but are not limited, to C.I. Disperse Orange 13, 29, 31:1, 33,49, 54, 55, 66, 73, 119 and 163; C.I. Disperse Red 4, 11, 54, 60 72, 73,86, 88, 91, 92, 93, 111, 126, 127, 134, 135, 143, 145, 152, 153, 154,159, 164, 167:1, 177, 181, 204, 206, 207, 221, 258, 278, 283, 288, 311,323, 343, 348 and 356; C.I. Disperse Violet 33; C.I. Disperse Blue 4,13, 56, 73, 113, 128, 148, 154, 158, 165, 165:1, 165:2, 183, 197, 201,214, 224, 225, 257, 266, 267, 287, 358, 359, 360, 379, Disperse Brown26, 27; and Disperse Yellow 5, 42, 54, 64, 79, 82, 83, 93, 99, 100, 119,122, 124, 126, 160, 184:1, 186, 198, 199, 204, 224 and 237. Depending onthe specific application, other organic and inorganic pigments, andsoluble and insoluble dyes, such as direct dyes, acid dyes, reactivedyes, vat dyes, cation dyes, basic dyes, luco dyes, thermochromatic, andphotochromatic colorants may also be used.

The colorant will remain as a particulate in order to be heat activatedor sublimed. This is not a significant issue for water insolublecolorants, such as sublimation dyes, in lower viscosity applicationswhere little to no glycol or other viscosity increasing agents arepresent. The difference between the boiling point of water and the inkheat activation temperature, which is typically greater than 50° F.,indicates that activation or sublimation of the ink solids will occurafter the aqueous components vaporize, so that activation or sublimationis not materially hindered by these components of the ink.

The relatively high concentration of high boiling point of glycols,polyol, and other viscosity control ingredients can create a highboiling system which makes the activation or sublimation of thecolorants very difficult. The boiling point may be near, or even above,the heat activation temperature of the dye, such as within 20° F. of theheat activation temperature. Particles with smaller diameters maytemporarily, or even permanently, bond with these ingredients and notactivate, due to hydrogen bonding, entrapment, or forming a high boilingpoint co-boiling system. Solvency of the non-polar portion of theseingredients may also contribute to the hindering of the colorant,especially the outside portion of the colorant particulates at atemperature near the boiling point of these solvents or agents. Only theinside portions of the colorant particles, which are not in contact withthe “bulk” ingredients of the ink, may activate or sublimate.

In one embodiment of the present invention the particulate size of theheat activated colorants are limited so that sufficient colorantmolecules are be successfully activated or sublimated. The followingmodel is indicative of the particles sizes of the dye:

$d \geq \frac{2{aT}_{s}}{1 - K^{1/3}}$

where,

-   -   d is the minimum particle size in diameter (in micron) to ensure        heat activation efficiency K    -   K is the heat activation efficiency (K<1)    -   Ts is the molecular size of heat activated colorant in the        longest dimension (in nanometer, generally Ts=1.25)    -   a is the solvency hindering impact parameter, for high viscosity        aqueous inks, a≧1        For relatively high viscosity inks having more than 20% by        weight of high viscosity solvent, a=3, meaning about three        layers of molecules of the colorant particulate may be hindered.        Therefore, the relationship between the particle size and heat        activation efficiency can be defined as:

$d \geq \frac{7.5}{1 - K^{1/3}}$

where,

-   -   d is the minimum particle size in diameter (in micron) to ensure        heat activation efficiency K    -   K is the heat activation efficiency (K<1)

One embodiment of the invention has a heat activation efficiency higherthan 65% (K) for the colorant particulates. Substantially all particlesof the colorant having a diameter of less than 50 nanometers areexcluded from the ink. The low transfer efficiency caused by the highconcentration of the viscosity control solvents/co-solvents to theseparticulates is thereby substantially eliminated.

The present invention may be used with inkjet inks for continuousinkjet, drop-on-demand thermal or bubble inkjet, drop-on-demand piezoelectric, ultrasonic or mechanical inkjet printhead delivery systems.Physical properties of the ink may be adjusted suitable for specificprinthead requirements. A preferred printer for printing an inkaccording to the invention is a RICOH GELSPRINTER inkjet printer, whichis designed to print OEM inks having a viscosity of about 7. Thisprinter is known in the industry as a printer for high viscosity inks.Inkjet printers designed for inks having a viscosity of 5.0 or greaterare preferred.

Heat activation is according to known processes for heat activation ofthe dyes. For example, a heat press may be used to activate and/ortransfer inks comprising sublimation dyes according to the teachings ofHale, U.S. Pat. No. 5,488,907.

The follow examples illustrate the general composition of the highviscosity heat activated ink.

Example 1

An ink for use with a Ricoh GelSprinter ink jet printer, nozzle size 35microns, and having a viscosity of about 7.5 cPs:

Ingredient Weight % Disperse blue dye (pre-stabilized) 3.5% Glycerin 40% Poly(2-ethyl-oxazoline)  2% Non-ionic Surfactant 3.5% Proxel GXL0.1% Other agents 2.0% De-ionized Water balance

The ink according to the example is produced with an upper limit (95%)of particles at 0.3 micron and a lower limit to 0.05 micron. Transferprinting of an image printed with the ink on polyester fabric using 400°F., with a 35 second heat activation time produces an image having anoptical density (cyan) of 1.25 or greater as measured by an X-Ritedensitometer.

Example 2

An ink for use with a Spectra Skywalker inkjet printer, nozzle size 45microns, and having a viscosity of about 15 cPs:

Ingredient Weight % Disperse dye mixture (pre-stabilized) 5.6%CAB-O-JET ® aq. Black pigment dispersion 1.5% Diethylene Glycol  20%e-caprolactam  15% Non-ionic Surfactant 3.5% Proxel GXL 0.1% Otheragents 2.0% De-ionized Water balance

This ink has a particle upper limit (95%) at 0.6 microns, and a lowerlimit to 0.05 microns. Direct printing onto polyester/cotton (50/50)with heat activation of the dye at 410° F., at 30 seconds, produces animage with an optical density of 1.30 or greater as measured by anX-Rite densitometer.

1. A method of printing a design by means of an ink jet printer usingheat activated colorant solids, comprising the steps of: preparing aliquid ink suitable for use in an ink jet printer which uses liquid ink,said liquid ink comprising water, heat activated colorant solids, and aviscosity control agent comprising not less than 15% by weight of theliquid ink, wherein the liquid ink has a viscosity of not less than 6.0centipoise, wherein the heat activated colorant solids are sparinglysoluble in the viscosity control agent, and wherein not less than 95% ofparticles of heat activated colorant solids have a diameter of 0.05microns or greater; supplying an ink jet printer that prints liquid inkwith said liquid ink; printing said liquid ink in a desired image bymeans of said ink jet printer on a substrate to form an image, whereinthe particles of the heat activated colorant solids are solid at thetime of printing; and heat activating the heat activated colorantsolids, wherein, after heat activation, the image has an optical densityof not less than 1.25 when measured by an X-Rite densitometer.
 2. Amethod of printing a design by means of an ink jet printer using heatactivated colorant solids, comprising the steps of: preparing a liquidink suitable for use in an ink jet printer which uses liquid ink, saidliquid ink comprising water, heat activated colorant solids, and aviscosity control agent comprising not less than 15% by weight of theliquid ink, wherein the liquid ink has a viscosity of not less than 6.0centipoise, wherein the heat activated colorant solids are sparinglysoluble in the viscosity control agent, and wherein an upper limit of adistribution of particle sizes of particles of the heat activatedcolorant solids is a function of both the smallest orifice of the printhead of an ink jet printer used to print the liquid ink and the ratio ofthe total weight of the head activated colorant solids to the totalweight of the liquid ink; supplying the ink jet printer that printsliquid ink with said liquid ink; printing said liquid ink in a desiredimage by means of said ink jet printer on a substrate to form an image,wherein the particles of the heat activated colorant solids are solid atthe time of printing; and heat activating the heat activated colorantsolids, wherein, after heat activation, the image has an optical densityof not less than 1.25 when measured by an X-Rite densitometer.
 3. Amethod of printing a design by means of an ink jet printer using heatactivated colorant solids, as described in claim 1, wherein the printerprints liquid ink having a viscosity of not less than 5.0 centipoise. 4.A method of printing a design by means of an ink jet printer using heatactivated colorant solids, as described in claim 2, wherein the printerprints liquid ink having a viscosity of not less than 5.0 centipoise. 5.A method of printing a design by means of an ink jet printer using heatactivated colorant solids, as described in claim 1, wherein theviscosity control agent increases the liquid ink to a viscosity of atleast 6.0 centipoise and the weight of the viscosity control agentpresent in a volume of heat activated ink is at least three times theweight of the heat activated colorant present in the volume of heatactivated ink.
 6. A method of printing a design by means of an ink jetprinter using heat activated colorant solids, as described in claim 2,wherein the viscosity control agent increases the liquid ink to aviscosity of at least 6.0 centipoise and the weight of the viscositycontrol agent present in a volume of heat activated ink is at leastthree times the weight of the heat activated colorant present in thevolume of heat activated ink.
 7. A method of printing a design by meansof an ink jet printer using heat activated colorant solids, as describedin claim 1, wherein the viscosity control agent is glycol and the glycolincreases the liquid ink to a viscosity of at least 6.0 centipoise andthe weight of the glycol present in a volume of heat activated ink is atleast three times the weight of the heat activated colorant present inthe volume of heat activated ink.
 8. A method of printing a design bymeans of an ink jet printer using heat activated colorant solids, asdescribed in claim 2, wherein the viscosity control agent is glycol andthe glycol increases the liquid ink to a viscosity of at least 6.0centipoise and the weight of the glycol present in a volume of heatactivated ink is at least three times the weight of the heat activatedcolorant present in the volume of heat activated ink.
 9. A method ofprinting a design by means of an ink jet printer using heat activatedcolorant solids, as described in claim 1, wherein the viscosity controlagent increases the liquid ink to a viscosity of at least 6.0 centipoiseand the weight of the viscosity control agent present in a volume ofheat activated ink is 15-60% of the total weight of the volume of heatactivated ink.
 10. A method of printing a design by means of an ink jetprinter using heat activated colorant solids, as described in claim 2,wherein the viscosity control agent increases the liquid ink to aviscosity of at least 6.0 centipoise and the weight of the viscositycontrol agent present in a volume of heat activated ink is 15-60% of thetotal weight of the volume of heat activated ink.
 11. A method ofprinting a design by means of an ink jet printer using heat activatedcolorant solids, as described in claim 1, wherein the heat activationtemperature of the colorant is not substantially higher than the boilingpoint of the viscosity control agent.
 12. A method of printing a designby means of an ink jet printer using heat activated colorant solids, asdescribed in claim 2, wherein the heat activation temperature of thecolorant is not substantially higher than the boiling point of theviscosity control agent.
 13. A method of printing a design by means ofan ink jet printer using heat activated colorant solids, as described inclaim 1, wherein the heat activation temperature of the colorant in notmore than 20 degrees Fahrenheit higher than the boiling point of theviscosity control agent.
 14. A method of printing a design by means ofan ink jet printer using heat activated colorant solids, as described inclaim 2, wherein the heat activation temperature of the colorant in notmore than 20 degrees Fahrenheit higher than the boiling point of theviscosity control agent.
 15. A method of printing a design by means ofan ink jet printer using heat activated colorant solids, as described inclaim 1, wherein substantially all particles of the colorant having adiameter of less than 50 nanometers are excluded from the ink.
 16. Amethod of printing a design by means of an ink jet printer using heatactivated colorant solids, as described in claim 2, whereinsubstantially all particles of the colorant having a diameter of lessthan 50 nanometers are excluded from the ink